1. Field of the Invention
The present invention relates to a photolithographic technique. More particularly, the present invention relates to a method of manufacturing a binary phase shift mask.
2. Description of the Related Art
Photolithography plays an important part in the manufacturing of semiconductors. Many processing operations such as etching and doping must go through photolithographic operations. In fact, the ultimate quality of most semiconductor devices depends on the resolution of light and depth of focus (DOF) in photolithography.
In the fabrication of semiconductors having a line width greater than 0.18 .mu.m, a binary photomask consisting of a quartz plate with a plated chromium film pattern thereon is often used. In general, a moderate to high-quality pattern can be transferred to a photoresist layer using this type of photomask.
However, when semiconductor products having a line width smaller than 0.18 microns are fabricated, light diffraction due to the reduction of hole diameters and line width in the mask pattern dominates. To minimize diffraction, a phase shifting layer is added to the binary photomask. Utilizing the positive and negative interference of light through the phase shift layers, resulting resolution of the photomask is improved. This type of photomask that also has an added phase shift layer is called a phase shift mask (PSM). The advantage of a phase shift mask is that resolution is improved without the need to use a new light source. Only changes in the photomask are required. A half-tone PSM is a photomask formed using phase-shifting material capable of shifting the phase of light by 180.degree.. In addition, the phase-shifting material is semi-transparent, and permits at most 30% of the light to pass through. In general, a portion of the pattern on the half-tone PSM is transparent. In other words, there is no phase shift through that area. Most phase shift materials contain molybdenum silicon oxy-nitride (MoSi.sub.z O.sub.x N.sub.y) and chromium oxide. However, a pattern to be transferred to a photoresist layer generally contains some small and some large openings. Moreover, some of the small openings may come very close to some large ones, for example, the layout of local interconnects. Consequently, side-lobe effects are a common occurrence.
FIG. 1A is a schematic cross-sectional view of a conventional half-tone phase shift mask. As shown in FIG. 1A, a phase shift layer 110 is formed over a quartz substrate 100. The phase shift layer 110 has a smaller opening 120 and a larger opening 130.
FIG. 1B is a graph showing the variation of luminosity on a photoresist layer during light exposure for a mask having cross-section as shown in FIG. 1A. As shown in FIG. 1B, the photoresist layer receives more light from the larger opening 130. In addition, intensity of light coming from the edge of the opening 130 is also greater (that is, diffracted light leaking out from the edge of the opening 130). Conversely, the photoresist layer receives less light from the smaller opening 120 and corresponding intensity from the edge of the opening 120 is smaller.
Since diffracted light coming from the edge of the opening 120 is relatively small, phase-reversed light from the phase shift layer 110 has sufficient intensity to cancel out the diffracted light. However, diffracted light coming from the edge of the larger opening 130 is more intense; hence phase-shifted light from the phase shift layer 110 is only able to cancel a portion of the diffracted light. Therefore, a swath of light is still emitted from the edge of the opening 130. Residual light from diffraction causes the exposure of photoresist in unwanted areas. This phenomenon is the so-called side-lobe effect.
This is a difficult situation because neither increasing nor decreasing light exposure helps. If the period of exposure is decreased, the photoresist layer receives too little light from smaller openings such as the opening 120. On the other hand, if the period of exposure is increased so that the photoresist layer receives sufficient light from all the smaller openings, side-lobe effects occur in all the larger openings such as the opening 130.